Refrigeration devices commonly used in existing air conditioning systems are chillers. Chilled water produced by a chiller passes through a channel and reduces ambient temperature by heat exchange. In recent years, chillers are widely used. One common type is a centrifugal chiller. The operating core is a centrifugal compressor. In order to save energy, multi-stage centrifugal compressors have become more common, but they exhibit non-proportionality in load control and poor capacity, adversely affecting control effects.
Referring to FIG. 1, the capacity-control performance of a traditional single-capacity centrifugal compressor is shown. As shown, when compared to the capacity-control performance of the system impedance line, this single-capacity centrifugal compressor (from overall flow 30% to overall flow 100%) cannot achieve a wide operation range for the system, so it is difficult for single-capacity centrifugal compressors to accomplish wide-range operations. In order to achieve wide-range operations, various multiple-capacity control methods are proposed. However, traditional multiple-capacity control methods usually involve adjusting a single inlet guide vane and a single diffuser. For simultaneous adjustments, only a fixed increment/decrement is provided. For sequential adjustments, one capacity-control mechanism is adjusted while the rest of the capacity-control mechanisms are unchanged, and another capacity-control mechanism is then adjusted only when the current one has reached its limit. Nonetheless, the above control schemes have less available control strategies and relatively poor control priority. As such, COP (coefficient of performance) is limited to be between 5.5 and 6.0, which only satisfies full-load efficiency, but not partial-load requirements, thereby reducing system efficiency and capacity.
U.S. Pat. No. 6,129,511 discloses a technique that controls only one set of an inlet guide vane and a diffuser by obtaining characteristic curves from actual measurements to know the relationships between the inlet guide vane and the diffuser and to establish a database thereof, thereby adjusting inlet guide vane in cooperation with the diffuser including inner and outer rings. Also, by measuring pressures, adjustment can be made through stepless control and interpolation, resulting in a compressor with high compression ratio. However, this type of control has less available variables and low flexibility. The overall control strategy is limited, which in turn limits the COP performance.
Moreover, U.S. Pat. No. 4,616,483 similarly adjusts a set of an inlet guide vane and a diffuser by controlling pressure values within a desired range in sequential increments or decrements based on measure current. Although this type of control method is simple and easy to use, it fails to provide wide-range operations and satisfy partial-load operations.
Furthermore, U.S. Pat. No. 5,807,071 similarly adjusts a set of an inlet guide vane and a diffuser. More specifically, the changes in the flow of refrigerant are controlled by the variable inlet guide vane in conjunction with rotating of inner and outer rings of the diffuser to turn on/off flow channel therein, thereby maintaining the compressor at peak efficiency, while suppressing surges. Also, the control is done sequentially based on the characteristic curves. However, this type of control has less available variables, and thus the overall control strategy is limited, which in turn limits the COP performance. It also fails to provide wide-range operations and satisfy partial-load operations.
From the above, it is clear that in the case of adjusting a set of an inlet guide vane and a diffuser in the prior art, it is difficult to achieve wide-range operations and satisfy partial-load operations. Moreover, the traditional multiple-capacity control techniques fail to provide a centrifugal compressor that improves the overall machine efficiency and suppresses surges. Thus, there is a need to provide a multiple-capacity centrifugal compressor and a control method thereof, which achieve proportionality in load control in the multi-stage centrifugal compressor and ensure wide-range operations, while increasing overall machine efficiency and reducing surges for safety and reliability.